FKBP12 is a small immunophilin that binds with high affinity to sarcoplasmic reticulum Ca2+ release channels (ryanodine receptors, RyRs) and transforming growth factor 21 receptors (T2I RI). In this application we are going to test the hypothesis that the gain of E-C coupling is regulated in a biphasic manner by FKBP concentration. Specifically we will: 1) Demonstrate that FKBP12 binding to RyR1 controls the gain of E-C coupling in a biphasic manner, thereby, modulating Ca2+ stores, force production, fatigue, and recovery from injury;2) Evaluate the ability T2RI activation to increase the gain of E-C coupling via FKBP12;3) Evaluate the ability of drugs that decrease FKBP12 binding to RyR1 to slow the development of fatigue and enhance recovery from injury and 4) Identify the amino acids on RyR1 that contribute to FKBP12 binding. The research described in this application will clarify the role of FKBP12 in skeletal muscle function and lay the groundwork for the development of new therapeutic interventions to slow diaphragm fatigue and enhance recovery from injury. PUBLIC HEALTH RELEVANCE: The research in this application will investigate the role of small immunophilins (FKBPs) in muscle Ca2+ homeostasis, fatigue, and recovery from injury. Diaphragm fatigue is a serious medical problem that contributes to respiratory failure in patients with skeletal muscle, cardiovascular and pulmonary diseases and to failure to wean patients from ventilators. Also, the rate of recovery of skeletal muscle from injury has profound effects on rehabilitation.